Fragmentation of network data will intensify iob consumption, if
the device receives a message storm of fragmented packets, the iob
cache will not be effectively used, this is not allowed on iot devices
since the resources of such devices are limited. Of course, this
also takes some disadvantages: data needs to be copied.
This option will brings some balance on resource-constrained devices,
enable this config to reduce the consumption of iob, the received iob
buffers will be merged into the contiguous iob chain.
Signed-off-by: chao an <anchao@xiaomi.com>